There is no longer any need to correlate data points across records to determine statistical information per request.
The overhead of turning on SMF 120.9 has been measured at approximately 1.5% of system CPU for the basic activity records.
The WP101342 Techdoc referenced below has an extensive description of the feature and was written by the architect and developer of the new function.
The degree of modification permitted is a function of the started task program and the limits of the environment.
The SMF 120.9 function of WAS z/OS supports the modify command with the following parameters: A sample Java program is available that will evaluate the dumped SMF 120.9 records and provide by a detailed and a summary view of the information they contain.